1. Field of the Invention
The present invention relates to a radio wave transmission cover, which is provided in front of a radar device for vehicles, and a method of manufacturing the radio wave transmission cover.
2. Description of the Related Art
Adaptive cruise controls (A.C.C.) are techniques, which measure the distance or a relative speed between a subject vehicle and a preceding vehicle, which is in front of the subject vehicle, using a sensor provided in the front end of the subject vehicle, and control a throttle or brake depending on the resultant data to accelerate or decelerate the subject vehicle, thus controlling a vehicle-to-vehicle distance. Recently, such Adaptive-cruise-controls have attracted considerable attention as a core technique for constructing an intelligent transportation system (ITS), which aims to reduce the incidence of traffic jams and accidents.
Typically, laser radar or millimeter wave (extremely high frequency wave) radar is used as the sensor for adaptive cruise controls. For example, millimeter wave radar transmits a millimeter wave, which has a frequency ranging from 30 GHz to 300 GHz and a wavelength ranging from 1 mm to 10 mm, to an object, and receives the millimeter wave, which has been reflected by the object. From this principle, the millimeter wave radar measures the distance or the relative speed between a subject vehicle and a preceding vehicle using the difference between the transmitted wave and the received wave.
Generally, radar devices for vehicles are disposed behind front grills of the vehicles. Such a front grill is uneven in thickness and is made of metal, or has metal-plated layers on the surfaces thereof, thus impeding the travel of radio waves. To overcome this problem, a technique in which a window part is provided in the front grill at a position corresponding to the front end of the radar device and a radio wave transmission cover made of resin is inserted into the window part has been proposed.
The radio wave transmission cover typically has a design layer for expressing various designs. The design layer is a relatively thin layer, which is formed through vapor deposition of metal or transferring of a film. Hence, it is required to cover each of the front surface and the rear surface of the thin design layer with a reinforcing resin layer (for example, refer to Japanese Laid Open Patent Publication NO. 2000-159039 and Japanese Laid Open Patent Publication NO. 2002-135030). FIG. 13 is a view schematically showing a representative example of a conventional radio wave transmission cover. Below, this conventional radio wave transmission cover will be explained with reference to FIG. 13.
To manufacture the radio wave transmission cover, in which front and rear surfaces of a design layer 103 are respectively covered with reinforcing resin layers 102 and 105, the first reinforcing resin layer 102 is first formed, thereafter, the design layer 103 is formed on the resin layer 102 through a vapor deposition, transferring or printing process. Subsequently, the second reinforcing resin layer 105 is formed on the design layer 103, thus completing the radio wave transmission cover.
However, in the conventional technique, because the design layer 103 is relatively thin, when the radio wave transmission cover is manufactured through an injection molding process, the design layer 103 may be deformed by the injection pressure, with which resin is injected to form the resin layer 105 on the design layer 103. Particularly, as shown in FIG. 13, there is the possibility of major deformation of a portion of the design layer 103 around an injection gate 170 of an injection mold 107. As such, if the design layer 103 is deformed, the design of the radio wave transmission cover suffers.
Meanwhile, a radio wave transmission cover may be manufactured by a method in which two reinforcing resin layers 102 and 105 are formed separately and are adhered to each other to form an integrated structure. However, in this case, an adhesive having the same (or almost the same) relative permittivity as that of the two reinforcing resin layers is required in order to adhere the two reinforcing resin layers to each other. If the relative permittivity of the adhesive differs greatly from the relative permittivity of the two reinforcing resin layers, the radio wave transmission loss of the radio wave transmission cover is increased, in other words, the radio wave transmissibility of the radio wave transmission cover is markedly reduced. Typically, polycarbonate or AES is mainly used as material for the reinforcing resin layers. However, the relative permittivity of a typical adhesive does not correspond to the relative permittivity of the above-mentioned resin. Hence, the method in which two reinforcing resin layers are formed separately and are adhered to each other such that they are integrated with each other is problematic in that it is difficult to ensure superior radio wave transmissibility of the radio wave transmission cover.
Furthermore, in the case where two reinforcing resin layers are formed separately, an air layer is formed in a gap between the reinforcing two reinforcing resin layers in the front-to-rear direction (hereinafter, referred to simply as a gap). If the gap is relatively large, the air layer formed between the two reinforcing resin layers is thick. Because the relative permittivity of air differs from the relative permittivity of the two reinforcing resin layers, if the air layer is relatively thick, the radio wave transmissibility is markedly reduced. Therefore, there is a disadvantage in that, if the gap between the two reinforcing resin layers is relatively large, the radio wave transmission cover has poor radio wave transmissibility.
Meanwhile, if the gap between the two reinforcing resin layers is excessively small, when the radio wave transmission cover expands or contracts at high or low temperatures, the two reinforcing resin layers may come into contact with each other. In the case where the design layer is provided between the two reinforcing resin layers and the radio wave transmission cover is configured such that the design thereof is determined by the design layer, if the design layer is damaged by the interference between the two reinforcing resin layers, the design of the radio wave transmission cover may suffer.
To prevent the two reinforcing resin layers from interfering with each other, it is preferable that the linear expansion coefficients of the two reinforcing resin layers and the adhesive match each other. As such, to obtain a radio wave transmission cover having superior radio wave transmissibility, the relative permittivities of the two reinforcing resin layers and the adhesive must match each other, and the linear expansion coefficients of the two reinforcing resin layers and the adhesive must also match each other. However, these characteristics cannot be easily realized. Therefore, it is very difficult to ensure superior radio wave transmissibility of the radio wave transmission cover.